Timing devices have been used in photographic darkrooms for such purposes as timing enlarger exposure times with sensitive photographic papers and for the precision performance of wet chemical process steps. The timing intervals used for the exposure of photographic papers are generally quite short, usually in the order of several seconds, whereas typical wet process steps, such as film and paper development, require time intervals on the order of one to several minutes.
In the past, a number of devices have been designed for aiding the measurement of time intervals in the darkroom. The most common solution to the problem of timing photographic processing steps is the spring-wound mechanical timer. Some of these timers are even provided with switches which allow them to control electrical appliances to which they are connected. However, these timers do not have any automatic means for stopping or canceling the timing period. These mechanical timers are of limited usefulness because they are generally limited to measuring time periods in either seconds or minutes. Perhaps even a more significant drawback experienced with mechanical timers is the fact that their inaccuracy often ranges as high as 10%. Moreover, although some of these devices have end alarms for sounding the end of a timing period, they do not include any means for aurally advising the user of the passage of time in the darkroom. This is especially important insofar as it is not possible to see the face of the timer in darkness. Nor is it possible to carry on an operation such as dodging or burning in a particular part of the photograph while looking at the face of the timing device.
Synchronous motor timers operating from the power line and using a conventional gear train may include a face for displaying the passage of time and means for actuating a relay. However, like spring-wound timers these timers usually time seconds or minutes functions, not both, and cannot conveniently be reset automatically. Also, the accuracy of these timers is in the order of 5%.
Still yet another approach to the problem of the timing of photographic operations is the use of an R-C network with decade switches selecting the resistance value for providing the desired timing interval in seconds for enlargement purposes. Accuracy in such a system may range between 2% and 5% depending upon the electrical circuit configuration and the precision of the components utilized. However, there is no convenient way of displaying time on these devices, and the nature of the R-C networks used makes precise timing difficult with increasing time intervals. Thus, using this sort of arrangement, the fabrication of a timer for timing periods in the order of one or several minutes is difficult and expensive to achieve with any degree of precision.